The present invention relates generally to devices for the measurement of peak values of a nonperiodic phenomenon having low rates of recurrence and in particular the application of said device to the indication of the state of discharge of a lead power battery.
The phenomenon that is desired to be measured is in the form (shown in FIG. 1), wherein a variable decreases in a discontinuous manner, as a function of time, in successive stages. The stages are furthermore separated by intervals of random duration, in the course whereof said variable decreases in an unsignificant fashion but while maintaining a value that is always less than that of the subsequent stages. Moreover, the duration of the said intervals is short with respect to the total duration of the decrease.
Let us consider the case of a decreasing potential (FIG. 1), in the manner mentioned hereinabove, from a value U max to a value U min during a relatively long period of time T (several hours) in n successive stages, U.sub.1, U.sub.2 . . . U.sub.n, separated by intervals of short duration t compared with T (of an order of tens of seconds, yielding a T/t ratio of the order of 5000), during which the potential assumes insignificant values that are lower even than U min. Direct measurements of such a potential by means of a voltmeter are thus excluded. One prior art solution to this problem consists of using an integrating voltmeter capable of damping the deviation of the needle, but the error of measurement is very large in such a case, on the order of 20 to 50%. Another solution consists of using a sampling method, i.e. to preserve the value of the preceding stage during the duration of the interval in order to take into consideration the new stage, making it necessary to know a second variable (intensity). Another approach to a solution consists therefore of accurately measuring the value of each stage and to determine, in the intervals, in the most accurate manner possible of the rate of decline of the last value measured, until the next value is determined. The exact rate of the decrease is given by the straight line d.sub.1, the slope .alpha. whereof may be expressed as follows: ##EQU1## wherein x is the total fractional decrease of the potential U and T is the total time for this decrease. It may be seen that if U has a value of several volts and the time T is several hours, the value of .alpha. will be very low, of the order of several microvolts per second.
Analog circuits having a time constant this high cannot be designed, and it is therefore necessary to utilize a numerical device which is expensive and bulky in order to monitor the rate of decline. If an analog circuit must be used, it must be able to operate with a lesser time constant and for this purpose, in the time interval t, a rate of decrease with a greater slope, in the form of the straight line d.sub.2, must be accepted. This greater slope is equivalent to accepting in each of these intervals a certain percentage of error y in the indication read. The slope of the straight line d.sub.2 is then described by the expression given hereinbelow, assuming that the decrease according to the slope .alpha., during an interval of time t, is very small: EQU .beta.=(yx/t)U=(yT/t).alpha.
Assuming a value of y of the order of 2% and of T/t of the order of 5000, the value of .beta. will be approximately 100 times higher than that of .alpha.. The time constant of an analog circuit capable of reproducing such a decrease thus would be of the order of tenths of millivolt/sec. A circuit having such a time constant is conceivable.